1. Field of the Invention
The present invention relates to apparatus and method for removing excess molten metal from a metallic strip by means of gas wiping after the strip has been lifted out of a bath used for plating the strip with molten metal.
The invention relates to plating of various metals, including but not limited to zinc, 5% Al zinc, 55% Al zinc and 100% aluminum, for example.
2. Description of the Related Art
In a continuous molten zinc plating line, for example, in which a steel strip is plated with zinc, excess molten zinc on the front and back surfaces of a steel strip is wiped away by jetting a gas from wiping nozzles onto the front and back surfaces of the steel strip. Reference is made to FIG. 8 of the accompanying drawings, wherein the steel strip is identified as xe2x80x9caxe2x80x9d and the wiping nozzles are xe2x80x9cbxe2x80x9d. In this manner, the amount of pickup of zinc to be plated on the steel strip is limited. This controls the excess molten zinc carried up from the bath, on the front and back surfaces of the steel strip a, when the strip is lifted from the molten zinc bath. However, such pickup control is confronted by the problem that the gas, having jetted from the wiping nozzles b, escapes outwardly of the steel strip a on its two side edges, causing so-called edge overcoat in which the zinc adheres in an excess amount to each edge of the steel strip a.
To cope with this edge overcoat problem, the present assignee Kawasaki Steel Corporation has previously proposed a gas wiping apparatus as disclosed in Japanese Unexamined Patent Application Publication No. 1-208441.
This prior wiping apparatus is constituted, as viewed in FIG. 9 of the drawings herewith, of wiping nozzles b of the aforesaid type; a pair of baffle plates c extending widthwise of the upwardly moving steel strip a and at a height covering a gas impingement point A, where gases jetted from the wiping nozzles L are caused to impinge on both the front and back surfaces of the steel strip a; and an edge wiping nozzle e disposed between each such baffle plate c at its inner edge and the steel strip a at its outer edge, as shown. The edge wiping nozzle c is provided with a gas jet d aimed downstream on the steel strip a of the gas impinging point A and in the direction of travel of the steel strip a. The edge wiping nozzle c is operated to direct a jet toward the widthwise direction on the steel strip a, the jet being caused to travel upstream and in parallel with the widthwise marginal edge of the steel strip a. By the arrangement of the baffle plate c, the two opposed gas streams jetted from the wiping nozzles a, aimed at both the front and back faces of the steel strip a, are prevented from interfering with each other at the position outwardly of the two side edges of the steel strip a. This prevents edge overcoat. Moreover, a gas jetted from the edge wiping nozzle d is aimed such that fine molten metal that is produced during wiping, which fine metal is called xe2x80x9csplash,xe2x80x9d is prevented from adhering to and depositing on and further growing on the baffle plate c located adjacent to the edge of the steel strip a, and molten metal is prevented from growing in bridge-like form between the baffle plate c and the edge of the steel strip a.
However, such conventional gas wiping apparatus has the drawback that it fails to adequately prevent edge overcoat and splash, depending upon the positioning of both the baffle plate and the edge wiping nozzle.
Accordingly, it is one object of the present invention to provide a gas wiping apparatus and method which is capable of preventing edge overcoat and splash with reliability.
We have examined various different ways of positioning a baffle plate and an edge wiping nozzle, and have discovered surprising phenomena.
As shown in FIG. 3 of the drawings, which shows only one of the two edges of the sheet 9, the distance between the gas jet port opening 71 of an edge wiping nozzle 7 and the gas impingement point A of face-wiping nozzles 2, 2xe2x80x2 may be designated L (mm), and the clearance between the outer edge 91 of the steel sheet and the inner edge 61 of a baffle plate 6 is designated C (mm). These distances and clearance can be accurately adjusted by the apparatus of this invention, as will further be described in detail hereinafter. We have newly discovered that a significant interaction is presented between L and C, which interaction is surprising and totally unexpected.
Namely, we have discovered that the optimum range of L is variable with the value of C. To sum up generally, L should become larger as C becomes smaller, whereas L should become smaller as C becomes larger.
The significance of the optimum range of C will now be explained. With regard to the baffle plate 6, it has been found that a C value of less than 4 mm causes splash to adhere to and deposit on the baffle plate 6 so that the molten metal is frequently apt to grow in bridge-like form between the edge of the steel strip 9 and the baffle plate 6. It has also been found that if C is more than 7 mm, the ratio of the edge spray pressure of the face spray pressure becomes too low, even if a powerful jet pressure-edge wiping nozzle is used. In this instance, molten metal cannot be sufficiently wiped away at the edges 91 of the steel strip, with consequent failure to prevent heavy edge overcoat. In addition, in some cases, splash adheres to and deposits on the baffle plate, even though the edges 91 of the steel sheet are spaced from their baffle plates 6.
Moreover, we have found that the spacing L is dependent upon the spacing C. In FIG. 4, there are shown the optimum interrelated ranges of L and C which we have discovered to be necessary to prevent edge overcoat and splash.
Note should be taken of the minimum value of L. When C is small, the minimum value of L should be large; otherwise the apparatus is incapable of preventing splash. For instance, when C is 7 mm, the minimum value of L must be 6 mm, and when C is 4 mm, the minimum value of L must be 12 mm. If L is maintained at 6 mm with C set at 4 mm, the drawback is encountered that splash re-adheres to and is deposited on the edge wiping nozzle, adhering once again to the widthwise marginal edge of the steel strip when the splash reaches a certain thickness. The drawback noted here cannot be overcome even when all possible adjustments are made to the gas jet quantities and gas pressures of the nozzle 7.
On the other hand, we have found that there is a maximum value of L. When C is large, the maximum value of L must be correspondingly small in order to prevent splash. For example, when C is 4 mm, the maximum value of L is 35 mm, and when C is 7 mm, the maximum value of L is 27.5 mm. If L is maintained at 35 mm with C set at 7 mm, the drawback arises that edge wiping becomes less effective so that splash occurring during wiping adheres to and deposits on the baffle plate and further grows thereon, or molten metal grows in bridge-like form between the baffle plate 6 (FIG. 3) and the edge 91 of the steel strip. Such drawback cannot be overcome, even when all possible adjustments are made to the gas jet quantities and gas pressures of the edge wiping nozzle 7.
With these surprising findings in mind, we have conducted further intensive researches and have discovered the important relationship between the clearance C (mm) and the distance L (mm) which enables edge overcoat and splash to be satisfactorily prevented. Thus, this invention has been made.
More specifically, the present invention provides a gas wiping apparatus and method wherein a plurality of face gas wiping nozzles extend widthwise of a strip material that is continuously conveyed upwardly from a liquid bath. The face gas wiping nozzles are aimed to direct jets of gases onto the front and back faces of the strip material, thereby limiting and controlling the pickup of the liquid deposited on the front and back surfaces of the strip material;
a pair of baffle plates disposed at a position extending from an edge of the strip material and at a location adjacent to the face gas impinging area on the faces of the strip material; and
an edge wiping nozzle disposed between the baffle plates at their inner edges and the edge of the strip material, the edge wiping nozzle being provided with a gas jet port positioned downward of the gas impinging point and in the direction of travel of the strip material, the edge wiping nozzle being operated to jet a gas toward the strip material traveling upstream and substantially parallel with the marginal edge of the strip material;
wherein a clearance C (mm) between the marginal edge of the strip material and the inner edge of the baffle plates is controlled within the range from 4 to 7 mm; and
when the distance between the gas jet opening of the edge wiping nozzle and the face gas impingement area is expressed as L (mm), the relationship between the distance L and the clearance C satisfies the following equation:
xe2x88x922.0C+20xe2x89xa6Lxe2x89xa6xe2x88x922.5C+45.